1. Field of the Invention
This invention relates to powering downhole electrical devices, and more particularly, to fuel cells that are adapted for downhole use in wells.
2. Description of the Related Art
More and larger electrical devices are being proposed for downhole applications. These include, for example, the use of electric motors for driving the drill bit and for driving downhole pumps for forward or reverse circulation of the drilling fluid. In large hole applications, such devices could be on the order of several hundred horsepower, with multiple devices used in the same downhole application. It is difficult, however, to transmit large amounts of power downhole for drilling purposes. In the static conditions associated with production environments, cables may be strapped to a production tubular, but even these hamper the initial deployment of the production string and more severely impact efficient workover operations. At high power levels, the size constraints placed on the cable size in the downhole environment leads to unacceptable power losses in the cable.
Other systems, such as wired drill pipe, suffer the same cable size constraints and are like wise unsuitable for transmitting large amounts of power downhole. In addition, such systems require complex surface connections, such as slip rings, with voltage levels that will cause considerable safety concerns. For wired drill pipe, literally hundreds of connections requiring multiple make/break cycles during the drilling of a well raises serious reliability concerns.
Batteries can be used as a local source of power for downhole electrical devices, but are subject to their own problems. For example, increasing the power and energy generation capacity of a battery generally requires a proportionate increase in the size of the battery, which can present difficulties given the space constraints that exist in wellbores. Also, batteries will typically need to be electrically recharged or replaced at some point.
Fuel cells make use of an electrochemical reaction involving a fuel and an oxidant in a cell that comprises an anode, cathode, and electrolyte, to generate electricity without also generating the unwanted by-products associated with combustion, while providing relatively higher energy efficiency. Thus, fuel cells potentially have a number of advantages over other power generation or storage means in many applications. A number of obstacles have hindered the use of fuel cells in high power and/or long term downhole applications. For instance, fuel cells typically provide reservoirs for the necessary fuel and oxidant, which without replenishment, limit the overall run time. Additionally, the reaction product, typically water, needs to be removed from the fuel cell stack in order to continuously run the fuel cell. Removal of the water downhole presents a challenge because the surrounding pressure is commonly higher than that present in a conventional fuel cell placed at surface in an ambient environment and operating in air. Using a pump to expel the water into the high pressure downhole environment may require a large amount of power.
VanBerg U.S. Pat. No. 5,202,194 describes a power supply for providing electricity to electrical circuits located downhole in a well. The power supply comprises a fuel cell, which is fed by hydrogen from a pressure container and oxygen from compressed oxygen gas bottles. Pressure regulators are located in the line between the hydrogen container and the fuel cell, and in the line between the oxygen bottles and the fuel cell. A pump is used to eject water from the fuel cell into the wellbore. The downhole deployment time is limited by the fuel and oxygen supply volumes.
There is a need for a downhole fuel cell that can provide substantial amounts of power over long durations.